1. Field of the Invention
This invention relates generally to methods for coating a substrate by cathode sputtering. More particularly, it relates to methods for coating a substrate by use of a magnetron sputtering device.
2. Prior Art
Sputtering techniques for depositing a coating on a substrate are well known. Sputtering is the physical ejection of material from a target by ion bombardment of the target. The ions are usually created by collisions between gas atoms and electrons in a glow discharge. The ions are accelerated into the target cathode by an electric field. A substrate is placed in a suitable location so that it intercepts a portion of the ejected atoms. Thus, a coating is deposited on the surface of the substrate.
Certain devices, known generally as magnetron cathodes, are useful in obtaining higher sputtering rates and higher quality coating deposits. In a magnetron cathode, a magnetic field is used to confine the glow discharge plasma and to increase the length of the path of electrons moving under the influence of the electric field. This results in an increase in the gas atom-electron collision probability. This leads to a much higher sputtering rate than obtained without the use of magnetic confinement. Further, the sputtering process can be accomplished at a much lower gas pressure.
One particular type of magnetron cathode allows the use of a flat plate target. Such a planar magnetron cathode is sold under the trademark "SPUTTER-RING" by Airco Temescal, Berkeley, California, a division of Airco, Inc. The construction of a related device is described more particularly in copending John S. Chapin application 438,482 filed Jan. 31, 1974. In this device, the glow discharge plasma is confined to an annular region which is parallel to the surface of the target and separated from it by about a millimeter. In operation, the magnetic confinement of the plasma results in a high rate of erosion in an annular region on the surface of the target. This erosion region is aligned with the glow discharge region. Looking at one segment of this annular erosion region, the ion bombardment and the sputtering rate is greatest in the center of the segment directly under the most intense region of the gas discharge. The ion bombardment decreases toward the outer and the inner edges of the erosion region.
With this planar magnetron device, a substrate can be rapidly covered with a metallic coating by using a dc potential to sputter a target plate of the desired metal in a chamber containing an inert gas. However, a severe problem is encountered when it is attempted to form a metal-oxide coating on a substrate by reactively sputtering a metal target in a chamber containing oxygen. This problem is continual formation of high current arcs in the system. These arcs immediately reduce the sputtering rate. Further, the greatly increased current may damage components in the power supply. It is, of course, possible to prevent component damage by a design which limits the current. However, it is necessary to reduce the current in order to extinguish the arc. Even if the power supply were designed to reduce the current and automatically restore it, the effective sputtering rate would inevitably be decreased. Further, when the current is restored, the arc would be likely to reignite and require a subsequent power supply shut-down. It is clearly desirable to prevent the formation of an arc in the first place.
As is known in the art of reactive sputtering, the reactant gas is established at a partial pressure dependant upon the desired results. Since reactive sputtering is a well-known technique, and since the particular partial pressures are not critical to the invention, this aspect of the method of the invention will not be described in detail. It is sufficient that the partial pressure of the reactant gas be such as to cause the desired reaction with the atoms of the metal being sputtered but not so high that the sputtering process is inhibited by gas scattering. In reactive sputtering, the reactant gas forms a compound with the material which is sputtered from the target plate. When the target plate is a metal, and the reactant gas is oxygen, a metal oxide is formed on the surface of the substrate. The metal oxide may be formed by a reaction at the surface of the target plate, in the region between the target plate and the substrate, or at the surface of the substrate. The fraction of the substrate coating which is formed in the various locations is not known.
Reactive sputtering is just one example of a coating process in which an insulating layer may be formed on a conductive target which is being bombarded with ions.